1. Field of the Invention
The present invention relates to a surface illuminant device which can be used as a backlight of a liquid crystal display device or the like.
2. Description of the Background Art
A conventional backlight of a liquid crystal display device or the like generally employs an edge light system. In the edge light system, a cold cathode fluorescent tube is used as light source and placed on a side (edge) of a light guide plate, an optical sheet is placed on a surface of the light guide plate from which light is emitted (light emission surface), and a reflecting material is placed on the bottom side of the light guide plate (the side opposed to the light emission surface). Efforts have been made to improve efficiency in use of light and accordingly enhance luminance by adjusting the shape of the light guide plate as well as the shape and type of the optical sheet. Further, efforts have been made to emit the light from the light emission surface in all directions as uniformly as possible.
In addition, efforts have been made to improve luminance in the direction normal to the light emission surface by using a prism sheet or the like according to use, so as to emit the light in the direction of the normal line in concentrative manner. Alternatively, efforts have been made to improve luminance in a specific direction by emitting light in the specific direction in concentrative manner.
Japanese Patent Laying-Open No. 8-262441, for example, discloses a surface illuminant element which emits light in the direction of normal line in concentrative manner by the structure constituted of a light source having directivity in two specific directions and a prism sheet arranged such that the apical angle of a prism faces downward. Japanese Patent Laying-Open No. 8-262442 discloses a surface illuminant element which emits light just in a desired direction in concentrative manner by employing the structure similar to that disclosed in Japanese Patent Laying Open No. 8-262441.
However, when the conventional technique is employed in the car navigation, for example, in which the liquid crystal display device is located at the central portion of the car and persons in the driver""s seat and the passenger seat watch the display device from two specific directions, a relatively large amount of light is emitted in directions other than the watching directions and thus improvements in the efficiency in use of the light are demanded in the conventional technique.
The present invention has been made to solve the problems above. One object of the present invention is to provide a surface illuminant device to emit light in a plurality of directions in concentrative manner and accordingly enhance luminance in the plurality of directions.
Another object of the present invention is to provide a surface illuminant device to emit light in two specific directions in concentrative manner and accordingly enhance luminance in the two specific directions.
Still another object of the present invention is to provide a surface illuminant device which has a luminance distribution (light distribution characteristic) with a gentle decrease in luminance in the vicinity of the direction in which the light is emitted in concentrative manner.
According to an aspect of the invention to accomplish the objects above, the surface illuminant device includes a surface illuminant which does not allow light to be emitted in a specific direction and a first optical element which has a plurality of prisms each having a predetermined apical angle. The first optical element is arranged on a light emission side (the side from which light is emitted) of the surface illuminant.
Preferably, the first optical element has one side and the other side opposite to the one side and the plurality of prisms are placed on the one side of the first optical element.
Still preferably, the one side of the first optical element on which the plurality of prisms are placed is opposite to the light emission side of the surface illuminant.
Preferably, the predetermined apical angle is 60xc2x0 to 94xc2x0.
Preferably, the plurality of prisms are a plurality of linear prisms elongating in a first direction each having a substantially triangular cross section, and they are arranged with their respective apical angles facing in a second direction which crosses the first direction.
Preferably, the plurality of prisms are arranged in a third direction which crosses the first and the second direction.
Preferably, the plurality of prisms are a plurality of linear prisms elongating in a first direction having a wave-shaped cross section, and they are arranged with respective apical angles facing in a second direction which crosses the first direction.
Preferably, the plurality of prisms are arranged in a third direction which crosses the first and the second direction.
Preferably, the surface illuminant device further includes a second optical element which condenses incident light.
Still preferably, the second optical element is arranged on a light emission side of the first optical element.
According to the present invention as described above, a surface illuminant device can be provided which emits light in a plurality of specific directions in concentrative manner to enhance luminance in the plurality of specific directions. Further, a surface illuminant device can be provided which emits light in two specific directions in concentrative manner to enhance luminance in the two specific directions. In addition, a surface illuminant device can be provided which has a luminance distribution (light distribution characteristic) with a gentle decrease in luminance in the vicinity of the direction in which light is emitted in concentrative manner.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.